National Rifle Association President David Keene said Tuesday that the organization intends to challenge the constitutionality of Maryland's newly passed gun law, as a conservative group readied plans to try to overturn the law through voter referendum.

Keene said during a radio interview the group will “absolutely” go to the courts.

“We are already in court in New York and we will be in court and aiding those in Maryland — and I am myself a Maryland resident — who want to challenge the constitutionality of this and other provisions here in Maryland,” Keene said to the Washington, D.C., station WTOP.

The gun bill approved by the General Assembly last week bans the sale of 45 types of military-style semi-automatic rifles, among other strict provisions. Keene said it is unconstitutional to ban firearms in common use and that the AR-15 is nation's best-selling rifle.

Meanwhile, a group that successfully petitioned three Maryland laws to referendum last year plans to submit to the state Board of Elections on Wednesday a proposal to put the gun law before voters.

Republican Del. Neil Parrott, founder of mdpetitions.com, said that while the group has not formally decided to fight the law at the ballot box, it hopes to have a petition ready by next week as a preliminary step. The group would need to gather 55,736 signatures to put an issue on the November 2014 ballot.

The gun law, which Gov. Martin O'Malley plans to sign during a ceremony in May, also limits magazines to 10 bullets and requires a license to buy a handgun. O’Malley told reporters Monday that he expected some of his agenda to be placed before voters.

“You can’t fear that,” O’Malley said of the referendum threat. “The people of our state are smart and they are fair.”

President Barack Obama called the governor to congratulate him on passing the gun law, according to O’Malley’s office. The Maryland legislation is among the toughest passed in any state legislature since the December shooting massacre of 27 at a Connecticut elementary school. Maryland’s law bans the AR-15 rifle that police said was used there.

The Maryland attorney general's office reviews every law before it is signed.

“We believe, as does the attorney general, that our new law is constitutional,” O'Malley spokeswoman Raquel Guillory said. “We currently have a ban on enumerated assault pistols, which has been on the books for decades.”

The new law is scheduled to take effect on Oct. 1. A successful petition drive could delay it for more than a year until voters decide the issue.

Del. Michael D. Smigiel, an Eastern Shore Republican and one of the most outspoken lawmakers on gun rights, said he expects out-of-state, pro-gun-control organizations such as New York City Mayor Michael Bloomberg’s Mayors Against Illegal Guns to “pour money into Maryland to uphold this” gun law.

“While we can’t match them in dollars, we will have many times the manpower out there,” Smigiel said.

Thousands of pro-gun advocates flooded Annapolis over the past three months to voice displeasure. On Tuesday some said they had not decided their next move.

“We are looking at our options,” said John H. Josselyn, legislative vice president for the Associated Gun Clubs of Baltimore.

The group posted a message to members on Sunday cautioning against a “knee-jerk” effort to petition the law.

“We have never won a referendum in Maryland, and it may not be the best course of action,” the message read. “We may be better off pursuing litigation in the courts.”

A ban on cheap handguns known as Saturday Night Specials was petitioned to referendum in 1988 and failed. At the time, there was less public outcry for new gun laws.